Structural Studies of Cytomegalovirus Entry Proteins

Abstract: Human cytomegalovirus (HCMV), a dsDNA, enveloped virus, is a ubiquitous pathogen that establishes lifelong latent infections in 50-90% of the population. It causes disease in persons with compromised immune systems, e.g., organ transplant recipients or AIDS patients. HCMV is also a leading cause of congenital viral infections and developmental defects in newborns. Entry of HCMV into cell... read mores requires the conserved glycoprotein B (gB), thought to function as a fusogen and reported to bind signaling receptors. gB also elicits a strong immune response in humans and induces the production of neutralizing antibodies, although most anti-gB antibodies are non-neutralizing. The crystal structure of the HCMV gB ectodomain determined to 3.6- resolution presented in this work is the first atomic-level structure of any betaherpesvirus glycoprotein. The structure of HCMV gB resembles the postfusion structures of HSV-1 and EBV homologs, establishing it as a new member of the class III viral fusogens. Despite structural similarities, each gB has a unique domain arrangement, demonstrating structural plasticity of gB that may accommodate virus-specific functional requirements. The structure illustrates how extensive glycosylation of the gB ectodomain influences antibody recognition. Antigenic sites that elicit neutralizing antibodies are more heavily glycosylated than those that elicit non-neutralizing antibodies, which suggest that HCMV gB uses glycans to shield neutralizing epitopes while exposing non-neutralizing epitopes to act as an immune decoy. This glycosylation pattern may have evolved to direct the immune response towards the generation of non-neutralizing antibodies thus helping HCMV to avoid clearance. The structure provided the framework for understanding the antigenic regions (AD1-5) of the protein, however, clinical information was needed to know which region would generate antibodies leading to protection in humans. By determining the level of antibodies against each antigenic region present in maternal sera from mothers of premature infants, we identified two trends affecting the ability of these antibodies to protect. The presence of antibodies against AD-5 correlated with protection from infection while the presence of antibodies against AD-1 interfered with protection. The latter was stronger and overcame the beneficial effects of anti-AD-5 antibodies. These trends await confirmation with larger sample sizes, yet they along with the HCMV gB structure offer valuable insights that may aid in the design of recombinant vaccines and monoclonal antibody therapies. read less